1. Field of the Invention
The present invention is generally in the field of electronic circuits and systems. More specifically, the present invention is in the field of electronic communications circuits and systems.
2. Background Art
Mixer circuits are widely used in communications systems to provide frequency conversion of a communication signal during signal processing in a receiver or transmitter. A mixer circuit may be utilized in a receiving system to down-convert a communication signal received at an antenna, from radio frequency (RF) to baseband, for example. When implemented in a receiver, a mixer typically works in conjunction with a low noise amplifier (LNA) which is used to boost an otherwise weak reception signal prior to down-conversion by the mixer. In many modern implementations, receivers, as well as transmitters, are employed in challenging environments, such as to support communication in a multi-mode wireless transceiver, for example. As a result, those systems may be exposed to significant levels of interference, and be expected to tolerate that interference without substantial deterioration in performance.
In the case of a receiver, interference can arise from several sources, including transmitters utilized in nearby wireless communication devices or base stations. Those transmitters may produce signals on the same frequency channel used by the receiver, or an adjacent frequency channel, and in many instances the interference signals may be substantially stronger than a desired reception signal. When large interference signals are present, an LNA providing a signal input to a mixer is likely to be forced to operate outside of its linear range. As is known in the art, non-linear operation of a receiver LNA can result in reception of “blocker signals” that degrade and interfere with a desired reception signal during signal processing. That phenomenon may be further exacerbated by the non-linear performance of the mixer itself.
A conventional approach to improving linearity and blocker resistance of a receiving system is to increase the linear range of the LNA receiving the antenna input. Because, as is known, blocker signals are not seen when an LNA is operating in its linear range, this approach may be effective in reducing LNA produced distortion of a reception signal. This approach fails to address interferences introduced by non-linearity of the mixer, however. Further, the blocker resistance is only effective during linear operation of the LNA, which comes at the cost of increased power consumption by the receiver. Moreover, interference may be seen across a wide dynamic range, and in the presence of powerful interference signals even an LNA with an extended linear range is susceptible to being driven out of linearity. In addition, the increased power consumption required to extend LNA linearity introduces disadvantages of its own, particularly for receivers implemented in mobile devices relying on a finite battery source for power.
Thus, there is a need in the art for a mixer circuit capable of providing frequency conversion of a communication signal, while displaying greater linearity than conventional mixer implementations, and improved blocker resistance over a wide dynamic range.